When an artificial material is implanted in a damaged region of a living body, the material is generally surrounded by, membranes of collagen fibers and thus isolated from neighboring bones. However, there have been known some artificial materials, which are not isolated by such fibrous membranes and strongly connect to bones in a living body. Examples of such artificial materials include Na2O—CaO—SiO2—P2O5-based bioglasses, sintered hydroxyapatite Ca10(PO4)6(OH)2, and crystallized glasses. Known as the crystallized glasses are, for example, CaO—MgO—SiO2—P2O5-based bioactive glasses containing wollastonite crystals and apatite crystals such as hydroxyapatite crystals. These materials are referred to as bioactive ceramics, and some of them have put into practical use as important bone restoration materials.
The sintered hydroxyapatites have been widely used in medical treatments as bone restoration materials with high biocompatibility, and production methods thereof have been widely studied. With demand for more biocompatible artificial bones, etc. increasing in recent years, however, it is desired to develop bioactive ceramics containing a carbonated apatite, a component of a bone in living body.
Because the carbonated apatites are lower in decomposition temperature than the hydroxyapatites, sintering is carried out at relatively low temperatures to provide carbonated apatite ceramics. JP 2000-72572 A discloses a molded implant produced by plastically working a sintered apatite body, and a method for producing the molded implant, which comprises the steps of sintering an apatite at 900° C. or lower, filling the sintered apatite in a predetermined mold, and plastically working the sintered apatite at 300 to 780° C. In this method, because the sintering temperature is low, a carbonated or fluorinated apatite with low decomposition temperature can be used to produce the implant having high biocompatibility. However, this implant mainly comprises the apatite without other crystal phases, thereby having low mechanical strength.
The use of glass as a sintering aid is known to increase the mechanical strength of the bone restoration ceramic material composed of the apatite such as the carbonated apatite. In the sintering process, the  glass is softened around main crystals of the apatite, and crystals are generated between the main crystals to be sintered, whereby the mechanical strength of the sintered apatite glass is increased. Conventionally, non-bioactive glasses are used as the sintering aid of the sintered hydroxyapatite body. However, because such non-bioactive glasses have high glass transition temperatures and/or crystallization temperatures, they cannot generate preferable crystals by sintering at temperatures lower than the decomposition temperatures of the carbonated apatites. Thus, the sintered carbonated apatite bodies using the non-bioactive glasses as sintering aids are not sufficient in the mechanical strength.